1. Field of the Invention
The present invention relates to methods and apparatus for measuring and testing the condition of composite materials. More particularly it relates to ultra-sound systems for determining and measuring stiffness degradation of fiber/matrix composite structures whether caused by moisture, heat or cold, radiation, chemicals, or any other source of degradation.
2. Description of the Prior Art
As composite materials become more widely used, especially in stiffness critical structural designs, such as tolerance sensitive high technology and aerospace systems, the ability to monitor the stability and integrity of the materials becomes more and more important.
U.S. Pat. No. 4,221,962 (Black, et al) discloses a method for detecting the presence of moisture, and thus degradation due to moisture, in the interior of fiber/matrix composite structures by embedding a radiation carrier, such as an optical fiber, inside of the structure, and then transmitting radiation through the radiation carrier. Changes in moisture in the structure effect the index of refraction of the radiation carrier, and thus provide a measurable indication of the moisture in and therefore the degradation of the structure due to moisture. However, the presence of moisture in a composite cannot always be associated with a degradation in a composite. Additionally the system of Black, et al requires the production of special composite materials, with an intrusive testing portion, and cannot be applied to composites in general.
R. D. Kriz and W. W. Stinchcomb, Elastic Moduli of Transversely Isotropic Fibers and Their Composites, J. Exp, Mech. 19, 41-49 (1979) demonstrated that it is possible to predict and measure the direction of energy flux propagation within a graphite/epoxy composite whose fibers are unidirectionally aligned at 45 degrees from the wave normal direction. However, this publication does not anticipate nor suggest measurement of changes in any such properties as an indication of the condition or of the degradation of such composites.
Other art includes: R. D. Kriz, Absorbed Moisture and Stress Wave Propagation in Graphite/Epoxy, Poster session presented at Defense Advanced Research Projects Agency/Air Force Materials Laboratory Review of Progress in Quantitative NDE, Aug. 2, 1981, Boulder, Colo.; and R. D. Kriz, Absorbed Moisture and Stress Wave Propagation in Graphite/Epoxy, Comp. Tech. Rev., Spring 1982; and R. D. Kriz, Monitoring Elastic Stiffness Degradation in Graphite/Epoxy Composites, Conference Proceedings for American Society for Nondestructive Testing, Boston, Mass., March, 1982; (Abstract concurrently in print, disclosing basic concepts and relationships now set forth in more detail and claimed in the present application).
Other than the latter three references, authored by the inventor of the present invention; none of the above cited prior art, nor any other known prior art, utilized, recognizes nor suggests monitoring changes in the direction of propagation or the deviation of energy flux transmitted into and through such composites.